The present invention relates to a process for producing nitric acid. More particularly, the present invention relates to a process for producing nitric oxide involving the supplemental injection of water and oxygen to provide a process exhibiting increased nitric acid production while permitting safe and efficient operation.
Nitric acid is generally manufactured on a commercial level by using a catalytic ammonia oxidation process. The process involves reacting ammonia and oxygen (generally obtained from air) over a catalyst, generally a platinum based gauze, in a reactor to selectively obtain nitric oxide and water: EQU 4NH.sub.3 +50.sub.2 .fwdarw.4NO+6H.sub.2 O
Generally, an excess amount of air relative to the stoichiometric amount is provided in order to control the flammability of the reaction mixture, the reactor outlet temperature and to provide extra oxygen for subsequent oxidation reactions.
The effluent gases from the reactor are then cooled in a series of heat exchangers to oxidize nitric oxide with oxygen to nitrogen dioxide and its dimer: EQU 2NO+O.sub.2 =2NO.sub.2 =N.sub.2 O.sub.4
The nitrogen dioxide and its dimer are then reacted with water in an absorption column to produce nitric acid. EQU 3NO.sub.2 (or 3/2 N.sub.2 O.sub.4)+H.sub.2 O.fwdarw.2HNO.sub.3 +NO
The resulting NO is oxidized again in the absorption column.
Several flow schemes exist for the catalytic ammonia oxidation process, but the following three zones are characteristic of all the processes. The three zones include a chemical combustion reactor, a chain of heat exchangers between the reactor and the absorption column, and the absorption tower.
The industry is constantly attempting to improve the production of nitric acid. Increased capacity, improvements in the product acid strength, as well as reduction of NO.sub.x effluent are all important objectives to rendering commercial processes more viable and acceptable. Steps have been taken in the prior art to primarily improve capacity.
For example, in U.S. Pat. Nos. 4,183,906 and 4,235,858, air enriched with oxygen is injected into the absorption tower. This injection of the enriched air allows excess air to be diverted to the front end reactor of the process to thereby increase capacity. It is also known, such as in U.S. Pat. Nos. 5,266,291 and 5,360,603, to inject air enriched with oxygen in the stream paths to the reactor. The available excess oxygen allows an increase in ammonia feed, thereby increasing the existing capacity. Generally, in such systems, the temperature of the system and flammability is controlled by water, carbon dioxide, nitrogen dioxide or nitrogen oxide injection into the front end of the reactor and/or into a specialized device of packed bed reactors. Such injection of enriched air to the reactor, however, creates a higher oxygen partial pressure, thereby resulting in an increase in the loss of catalyst by oxidation.
Increasing the production of nitric acid, particularly in existing facilities, is also an important objective. Such increased production, however, must be accomplished safely and efficiently.
The production increase of an existing nitric acid production facility, called debottlenecking, generally imposes the necessity of increasing the ammonia flow rate. At the same time less primary air is available, while more secondary air has to be sent to the stripping column to ensure good bleaching of the increased nitric acid produced. These two modifications in flow rates cause the reactor feed composition to change and the following two difficulties are introduced.
First, the ammonia percentage in the reactor feed, defined as ##EQU1## increases, which directs the operating conditions of the reactor towards the flammability zone, approaching the lower explosion limit (LEL) of ammonia in air.
Second, the reactor's outlet temperature rises, while less heat sink (e.g. inerts such as nitrogen, excess oxygen, and water) is present in the reactor feed per mole of ammonia. The reactor outlet temperature should not rise, as the catalytic gauze must be operated at the optimum gauze temperature.
In increasing the production of nitric acid in an existing facility, therefore, the problem is to increase the nitric acid production rate, while respecting the constraints of good product bleaching, safely operating out of the flammability zone and avoiding any increase in the reactor's outlet temperature. Efficient process operation to avoid catalyst loss due to volatilization is also an important consideration.
It is therefore an object of the present invention to provide an improved process for the production of nitric acid which enjoys increased nitric acid production capacity.
It is another object of the present invention to provide such an improved process which also avoids the problems of flammability and catalyst loss.
Yet another object of the present invention is to provide an improved process for the production of nitric acid which also respects the constraints of good product bleaching and avoids any increase in the reactors outlet temperature.
Yet another object of the present invention is to provide a novel nitric acid production process which can achieve the foregoing objectives using conventional equipment.
These and other objects of the present invention will become apparent upon a review of the following specification, the FIGURE of the drawing and the claims appended hereto.